As of version 6.28.00, GAM uses the People API to manage user contacts rather than the Contacts API.
Most commands will work unchanged but Google has completely changed how the data is presented. If you have scripts that process the output from print contacts for example, they will have to be changed.
You might want to keep an older version of GAM available so you can compare the output from the two versions and make adjustments as necessary.
If you manage contacts in the contact group "Other Contacts", you will need to use an older version, as the People API has very little support for this.
To use these commands you must add the People API to your project and authorize the appropriate scopes: